This invention relates to single-mode fiber optic couplers that are capable of coupling light between fibers with low excess loss.
Fused fiber couplers have been formed by positioning a plurality of fibers in a side-by-side relationship along a suitable length thereof and fusing and tapering the claddings together to secure the fibers and reduce the spacings between the cores. Various coupler properties can be improved by forming an "overclad coupler" in which the fibers are embedded in a glass matrix. An overclad coupler is formed by inserting a plurality of optical fibers into a glass tube and collapsing the tube midregion onto the fibers. The central portion of the midregion is thereafter drawn down to that diameter and coupling length which is necessary to obtain the desired coupling. The refractive index n.sub.3 of the inner region of the tube adjacent the fibers must be lower than the refractive index n.sub.2 of the fiber cladding. Commercially available single-mode optical fibers usually have a value of n.sub.2 that is equal to or near that of silica. If silica is employed as the base glass for the tube, a dopant such as B.sub.2 O.sub.3, and optionally fluorine, is added thereto for the purpose of decreasing the tube refractive index n.sub.3 to a value lower than n.sub.2.
When the tube collapses onto the fibers, the fibers are distorted so that their cross-sectional shape is no longer round. Although some amount of distortion always occurs, the degree to which the fibers are distorted depends upon the relative viscosities of the fibers and the tube at the temperature of the tube collapse step as well as upon the temperature profile of the preform, the vacuum level during the collapse step and the glass geometry. The coupler function may dictate the refractive index and thus the hardness of the tube glass. For example, U.S. patent application Ser. No. 07/913,390 now U.S. Pat. No. 5,268,979 (D. L. Weidman-6) filed Jul. 15, 1992, now U.S. Pat. No. 5,268,979 teaches an overclad achromatic fiber optic coupler of the type wherein a plurality of single-mode optical fibers are fused together along a portion of their lengths to form a coupling region that is surrounded by a matrix glass body of refractive index n.sub.3. The coupler taper and n.sub.3 are such that the coupling constants of the coupler at two widely separated wavelengths are identical, thus giving achromatic performance. The value of .DELTA..sub.2-3 is preferably chosen so that nonadiabatic taper excess loss is kept below 0.5 dB. A discussion of nonadiabatic taper devices appears in the publication, W. J. Stewart et al., "Design Limitation on Tapers and Couplers in Single-Mode Fibers", Proc. IOPOC, 1985, pages 559-562. In order to meet this requirement, it appears that .DELTA..sub.2-3 must be lower than 0.125% and preferably lower than about 0.02%. The value of .DELTA..sub.2-3 is obtained from the equation .DELTA..sub.2-3 =(n.sub.2.sup.2 -n.sub.3.sup.2)/n.sub.2.sup.2. The term .DELTA. is often expressed in percent, i.e. one hundred times .DELTA.. As .DELTA..sub.2-3 becomes smaller, less refractive index-decreasing dopant is present in the silica-based matrix glass tube. The relatively hard matrix glass tube that is required in the process of making this type of achromatic coupler deforms the fibers therein during the tube collapse step of the coupler forming process.
The cores become so small in the coupling region that their effect on propagation becomes very small. When the fiber cladding diameter becomes sufficiently small, the composite of the core and cladding functions as the light guiding portion of the waveguide in the coupling region, and the surrounding low index matrix material functions as the cladding. Power therefore transfers between the adjacent fiber claddings in the coupling region.
As the output fibers become distorted or flattened, the area of contact between adjacent fibers increases and the composite structure of all output fibers is able to support composite modes. In a 1.times.8 coupler, for example, eight output fibers are disposed around the input fiber. Computer modeling of the coupling in the 1.times.8 geometry shows that input light that is propagating in the central fiber will not completely transfer to the eight individual output fibers. Rather, a small portion will couple to modes of the composite structure of the eight output fibers; those modes do not couple completely to the fundamental modes of the output fibers (the lowest order such mode in a 1.times.8 coupler, for example, is a "ring" or "donut" mode which radially uniform about the propagation axis). Light in the ring modes does not completely transfer to each of the individual output fibers in the up-taper region, and excess loss increases.
Even in a fused biconically tapered coupler, in which no overclad tube is employed, surface tension causes the fibers to fuse together with large areas of contact between adjacent fibers. Therefore, this type of coupler is also subject to increased excess loss due to composite modes.